This invention relates generally to fasteners for relatively soft materials with elastic-plastic properties, such as wood and wood products, and more particularly to fasteners such as nails for creating joints in wood.
Typical fasteners used in soft materials with plastic-elastic properties, such as wood and wood products, involve forcing or screwing the fastener into the material, and create a joint bond through friction and interlocking between the fastener and the parent material. A widely used fastener for wood is a common round cylindrically shaped nail. A nail, spike, or other similar type of forced fastener is typically a device of relatively uniform geometric proportions having an elongated shank with a head and a pointed end. Commonly, the shank is cylindrically shaped and distinguished by parallel sides and substantially unvarying congruent cross section over its full length. The term “congruent” means that cross sections taken at different points along the shank length are substantially identical in form and fit one within the outline of the other. The shank, which is driven into a parent material, causes a portion of the material to compress plastically as it expands and creates a space that conforms to the shank. The compressed material deforms into the surrounding material, causing the surrounding material to be compressed elastically and to exert pressure on the fastener shank. The term “plastic deformation” refers to a deformation that is substantially permanent and irreversible, whereas the term “elastic” refers to a characteristic of material that has elasticity and tends to return to its original form when a stress is removed. In the case of a fastener shank that is driven into wood, there is a plastic deformation of the wood material caused by the shank that is irreversible, and a hole is left in the material as though it were drilled.
Where the shank plastically deforms the parent material, the displaced material around the shank is placed in compression and acts to grip the shank, exerting friction and resistance to lateral movement, or wobble, of the shank in the formed hole. Although the permanent deformation of the material is intrinsically weakening, some deformation is essential to forming a friction bond with the shank.
As the shank of the fastener accepts load, either transverse or parallel to the longitudinal axis of the shank, both the shank and the parent material in the vicinity of the shank experience an initial reaction that is elastic (up to a given load), meaning that when the load is removed the shank will return to its original configuration in the parent material. The hardness and elastic modulus of the typical fastener used for soft materials, such as wood, are generally greater than that of the parent material since the fastener is nearly always driven into the parent material and must create its own entry hole. The elastic range of the joint, where stress and strain are approximately proportional, is influenced by the elastic modulus of the weaker element (the wood or other parent material) and the local unit stress. Unit stress is determined by the area of the parent material affected, so that the greater the area for a given load the less the unit stress. For example, in the case of a fastener, the area may be the width or the diameter of the shank over a unit of shank length.
The holding power of fasteners such as nails is generally assumed to be a function primarily of the friction between the fastener and the parent material, and it is assumed that holding power increases with the depth of penetration of the fastener beyond a joint interface. This is usually expressed as a depth of penetration of the point in the joint component receiving the point, and is a designation determined by standard testing criteria. In practice, this turns out to be approximately a distance of eleven shank diameters from the joint. Thus, the traditional approach places emphasis upon the length of the shank as the shank acts like a beam developing end moments. However, there is a point where greater penetration does not produce greater holding power, and holding power is unaffected by penetration of the shank beyond that point. The most commonly used fasteners have bases or sides that are essentially parallel or developed along parallel planes, and form a generally prismatic section. These prismatic shapes, which are generally uniform in cross section, exhibit a uniform strength and resistance to bending over the length of the shank.
Common round cylindrical wire nails have several well-known disadvantages. They are wasteful of fastener material, e.g., steel, and use an excess amount of fastener material for the amount of holding power they afford. A round cylindrical nail has the smallest surface area for the given amount of fastener material used, which increases the cost of the nail, and this geometry impacts its effectiveness because of the lack of complete frictional contact between the shank and the material. The common round nail also has a tendency to wedge the wood apart (and may sometimes split the wood) which decreases the holding power by reducing the amount of surface area of the nail which is in contact with the wood. Furthermore, as parent material is irreversibly crushed and fails plastically, a fastener is deprived of support along its length and is subjected to increased bending strain. Both the fastener and the parent material may eventually experience plastic failure, which progresses until the joint separates in ultimate failure. Various approaches have been used to extend the range of elastic or quasi-elastic behavior between the fastener and the parent material. These include hardening the shank, roughening and ribbing a shank, using square shanks, bent shanks, crimped ends, cold riveted spikes, expanding collars and deformed shanks. Other methods have included “setting” the shank, after introduction into material, by further deforming it into the material as by expanding or crimping.
Some approaches to avoid the disadvantages of the round cylindrical nail have included fabricating the shank with grooves and ridges, such as annular grooves or fins that project radially outward from the shank. These ridges and fins have as their purpose increasing the withdrawal force required to remove the shank from the wood by embedding themselves into the wood transversely to the axis of the shank. Still other approaches have included fabricating the shank with a plurality of longitudinal fins that project radially from the shank to impart to the shank a finned or ridged cross section as shown, for example, in U.S. Pat. No. 7,089,403 to Seace or the star-shaped cross section shown in U.S. Pat. No. 4,973,211 to Potucek, or to give the shank a Y-shaped or cross-shaped cross section such as shown in U.S. Pat. No. 5,143,501 to Leistner and U.S. Pat. No. 4,637,768 to Rabe. The purpose of such projecting ridges and fins is either to reduce the amount of fastener material used relative to a common cylindrical nail, or increase holding power by increasing the amount of surface area of contact between the shank and the parent material. However, such known shanks having fins or ridges and grooves have not been effective in affording increased joint strength or reducing the size of the fastener.
For instance, the five-pointed star fastener of Potucek has relatively thin fins that press into distorted and crushed wood fibers. They are unsupported by the wood and function as weak cantilevers which are likely to deform under pressure. Although this fastener has a moment of inertia and section modulus that are greater than a conventional cylindrical fastener, the fins are structurally weak and relatively poor in transmitting loads to the overall structure. They are likely to fail or distort individually and undermine the shank as a structure in a manner similar to flange instability on a non-compact girder.
In other arrangements, such as the four grooved cross configuration of Leistner, increased holding power is stated to be the result of increasing the shank diameter by deforming the shank to produce the projecting ridges. The ridges of the Leistner fastener have a peripheral surface that is greater than 38% and may be as high as 50% of the increased circumference of the shank, and Leistner teaches that to obtain the increased holding power the shank should be inserted with the ridges transversely across the grain of the wood. The y-shaped configuration of Rabe must also be aligned with the grain of the wood. These fasteners which require some form of alignment of the shank with the material are impractical.
It is desirable to provide fasteners and wood products that afford greater holding power for a given shank diameter and length, which efficiently utilize shank material in a cost effective manner, which can easily withstand the load of being driven into wood and similar materials, and which does not require any particular alignment of the fastener with the grain of the wood material. It is desirable to provide a fastener which satisfies these criteria and which overcome other disadvantages of known fasteners, and it is to these ends that the present invention is directed.